Speech Processing of the Letter ‘zha’ in Tamil Language with LPC

Contemporary Engineering Sciences, Vol. 2, 2009, no. 10, 497 - 505
Speech Processing of the Letter
‘zha’
in Tamil Language with LPC
A. Srinivasan1 , K. Srinivasa Rao2 , D. Narasimhan3 and K. Kannan4
1
Department of Electronics and Communication Engineering,
2
DST Chair,
3,4
Department of Mathematics,
Srinivasa Ramanujan Centre, SASTRA University,
Kumbakonam - 612 001 India
E-Mail :1 [email protected]
3
[email protected] and 4 [email protected]
Abstract : Wideband speech signals of the letter ‘zha’
in Tamil language of
3 males and 3 females were coded using an improved version of Linear Predictive
Coding (LPC). The sampling frequency was at 16 kHz and the bit rate was at 15450
bits per second, where the original bit rate was at 128000 bits per second with the
help of wave surfer audio tool. The quality of the performance is exhibited through
the block diagram voice coder. In the last section, the tradeoffs between the bit rate
for a plain LPC vocoder and the bit rate for a voice-excited LPC vocoder with DCT
is analyzed.
Keywords : Speech processing, LPC, Tamil Language, Letter ‘zha’
1
, Wavesurfer.
Introduction
Tamil is one of the oldest and official languages in India. In Tamilnadu it is
the prominent and primary language. It is one of the official languages of the union
territories of Pondicherry and Andaman & Nicobar Islands. It is one of 23 nationally
recognised languages in the Constitution of India. It has official status in Sri Lanka,
Malaysia and Singapore. The art and architecture of the Tamil people encompasses
some of the notable contributions of India and South-East Asia to the world of art.
With more than 77 million speakers, Tamil is one of the widely spoken languages of
the world.
Tamil vowels are classified into short, long (five of each type) and two diphthongs.
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A. Srinivasan, K. Srinivasa Rao, D. Narasimhan and K. Kannan
Consonants are classified into three categories with six in each category: hard, soft
(a.k.a nasal), and medium.
The classification is based on the place of articulation. In total there are 18
consonants. The vowels and consonants combine to form 216 compound characters.
Placing dependent vowel markers on either one side or both sides of the consonant
forms the compound characters.
There is one more special letter āytham used in classical Tamil and rarely found
in modern Tamil. In total there are 247 letters in Tamil alphabet. In these 247
letters ‘Zha’ is the most significant, because of its usage and pronunciation. Many
people will not pronounce the letter ‘Zha’ properly. There are two letters with same
sound as ‘Zha’ (la, lla), so it is necessary to recognize the letter ‘Zha’.
2
2.1
Vowels and Consonants
Vowels
There are 12 vowels in Tamil, called uyireluttu (uyir - life, eluttu - letter). These
vowels are classified into short (kuril) and long (five of each type) and two
diphthongs, /ai/ and /au/, and three “shortened” (kurriyl) vowels.The long vowels
are about twice as long as the short vowels. The diphthongss are usually pronounced
about 1.5 times as long as the short vowels.
2.2
Consonants
Consonants are known as meyyeluttu (mey-body, eluttu-letters) in Tamil. It is
classified into three categories with six in each category: vallinam (hard), mellinam
(soft or Nasal) and itayinam (medium).Unlike most Indian languages, Tamil does
not distinguish aspirated and unaspirated consonants. In addition, the voicing of
plosives is governed by strict rules in centamil (Pure Tamil). Plosives are unvoiced if
they occur word-initially or doubled. Elsewhere they are voiced, with a few becoming
fricatives intervocalically. Nasals and approximants are always voiced.
Speech processing of the letter ‘zha’ in Tamil Language with LPC
499
As common place in languages of India, Tamil is characterised by its use of more
than one type of coronal consonants. Retroflex consonants include the retroflex
approximant /‘zha’
/ (H) (example Tamil), which among the Dravidian languages
is also found in Malayalam (example Kozhikode), disappeared from Kannada in
pronunciation at around 1000 AD (the dedicated letter is still found in Unicode),
and was never present in Telugu. Dental and alveolar consonants also contrast with
each other, a typically Dravidian trait not found in the neighboring Indo-Aryan
languages. In spoken Tamil, however, this contrast has been largely lost, and even
in literary Tamil, e and d may be seen as allophonic.
A chart of the Tamil consonant phonemes in the International Phonetic
Alphabet follows. Phonemes in brackets are voiced equivalents. Both voiceless
and voiced forms are represented by the same character in Tamil, and voicing is
determined by context. The sounds /f/ and /§/ are peripheral to the phonology
of Tamil, being found only in loanwords and frequently replaced by native sounds.
There are well-defined rules for elision in Tamil, categorised into different classes
based on the phoneme which undergoes elision.
3
Special letter- Āytam
Classical Tamil also had a phoneme called the Āytam. Tamil grammarians of the
time classified it as a dependent phoneme (or restricted phoneme) (cārpeluttu),
but it is very rare in modern Tamil. The rules of pronunciation given in the
T olkāppiyam, a text on the grammar of Classical Tamil, suggest that the āytam
could have glottalised the sounds it was combined with. It has also been suggested
that the āytam was used to represent the voiced implosive (or closing part or the
first half) of geminated voiced plosives inside a word.The Āytam, in modern Tamil,
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A. Srinivasan, K. Srinivasa Rao, D. Narasimhan and K. Kannan
is also used to convert pa to f a (not the retroflex zha [`]) when writing English
words using the Tamil script.
4
LPC Vocoder
The detailed LPC speech coding technique is viewed the specific modifications,
additions are given to improve this algorithm. However, before jumping into the
detailed methodology of our solution, it will be helpful to give a brief overview of
speech production. Sounds of speech are produced when velum is lowered to make
it acoustically coupled with the vocal tract. Nasal sounds of speech are produced
in this way. Speech signals consist of several sequences of sounds. Each sound can
be thought of a unique information. Generally the speech sounds are classified into
two types namely, voiced and unvoiced. The fundamental difference between these
two types of speech sounds comes from the way they are produced. The vibrations
of the vocal cords produce voiced sounds. The rate at which the vocal cords vibrate
dictates the pitch of the sound. On the other hand, unvoiced sounds do not rely on
the vibration of the vocal cords. The unvoiced sounds are created by the constriction
of the vocal tract. The vocal cords remain open and the constrictions of the vocal
tract force air out to produce the unvoiced sounds.
Figure 1: LPC Vocoder
LPC technique will be utilized in order to analyze and synthesize speech signals.
This technique is used to estimate the basic speech parameters like pitch, formants
and spectra. A block diagram of an LPC vocoder can be seen in Fig.1. The principle
behind the use of LPC is to minimize the sum of the squared differences between
the original speech signal and the estimated speech signal over a finite duration.
This could be used to obtain a unique set of predictor coefficients. These predictor
coefficients are normally estimated in every frame, which is normally 20 ms long.
The predictor coefficients are represented by ak . The synthesis filter takes the error
signal as an input and it is filtered and the output is the speech signal. The transfer
function of the time-varying digital filter is
H(z) =
1−
G
p
P
ak z −k
k=1
where, G is gain. For LPC-10 algorithm p is 10 and p is 18 for the improved
algorithm. The two most commonly used methods to compute the coefficients are,
Speech processing of the letter ‘zha’ in Tamil Language with LPC
501
the covariance method and the auto-correlation formulation.
For our
implementation, we will be using the auto-correlation formulation. However, if the
frame is unvoiced, then white noise is used to represent it and a pitch period of T=0
is transmitted. Therefore, either white noise or impulse train becomes the excitation
of the LPC synthesis filter. It is important to re-emphasize that the pitch, gain and
coefficient parameters will be varying with time from one frame to another.
5
Analysis of ‘Zha’ using WaveSurfer
WaveSurfer is a simple but powerful interface. The sound can be visualized and
analyzed in several ways with the help of this tool. In addition, a spectrum window
can be opened using Popup Spectrum Section for analyze Spectrum section plot
(Magnitude Vs Frequency). Further the special control windows are available for
Waveforms and Spectrograms, which allow the user to make quick modifications
such as sound edit, noise elimination etc. The basic document we work with is
sound files of 3 male and 3 female speakers with letter ‘zha’. The standard speech
analysis of the letter ‘zha’ such as Waveform, Spectrogram, Pitch, and Power panes
are analyzed and the samples are shown in following figures.
Figure 2: Waveform of letter ‘zha’
Figure 3: Spectrogram of letter ‘zha’
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A. Srinivasan, K. Srinivasa Rao, D. Narasimhan and K. Kannan
Figure 4: Pitch panes of letter ‘zha’
Figure 5: Power panes of letter ‘zha’
6
Experimental Results
The variation of the Spectrum section plot in LPC is measures for the letter ‘zha’ is
analyzed with the following parameters and the sample of magnitude Vs frequency
plot is shown in figure 6 and figure 7.
• Analysis type: LPC
• Analysis order: 20
• Speech signal bandwidth B = 8 kHz
• Sampling rate Fs = 16000 Hz (or samples/sec.)
• Channel: All
• Window type: Hamming
• Window length (frame): 512 points (20ms)
• Number of predictor coefficients of the LPC model = 18
6.1
Spectrum plot
Sample spectrum section plot of letter ‘zha’
Figure 6: Sample 1
Speech processing of the letter ‘zha’ in Tamil Language with LPC
503
Figure 7: Sample 2
6.2
Sl.No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Magnitude and frequency comparison of 3 male and
3 female speakers
Frequency
(Hz)
15.625
140.625
390.625
640.625
1015.625
2046.875
3140.625
4203.125
5234.375
5703.125
6140.625
7171.875
7953.125
7984.375
†F1
(dB)
-20.47
-32.68
-36.63
-41.36
-51.20
-51.68
-56.56
-61.15
-73.12
-72.42
-76.70
-73.45
-84.67
-84.74
†F2
(dB)
-20.98
-32.73
-36.74
-34.99
-47.91
-53.12
-58.12
-68.51
-70.22
-68.77
-71.34
-72.08
-84.46
-84.52
†F3
(dB)
-19.67
-32.01
-36.13
-40.97
-51.00
-51.02
-56.10
-61.12
-72.97
-72.27
-76.61
-73.39
-84.51
-84.57
††M1
(dB)
-21.02
-32.94
-36.94
-41.48
-51.90
-52.02
-57.00
-61.66
-73.92
-72.95
-77.13
-73.99
-85.23
-85.56
††M2
(dB)
-21.86
-33.13
-37.14
-41.92
-52.65
-52.98
-57.60
-61.98
-74.48
-73.25
-77.89
-74.19
-86.00
-86.02
††M3
(dB)
-20.71
-32.75
-37.01
-40.99
-50.90
-53.05
-56.28
-63.22
-73.62
-72.99
-77.69
-73.16
-85.92
-85.83
Table 3: Magnitude and frequency comparison of 3 male and 3 female
speakers. † Female, †† Male
6.3
Bit rates
The bit rate for a plain LPC vocoder and the bit rate for a voice-excited LPC
vocoder with DCT is calculated and shown in Table 4 and Table 5.
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A. Srinivasan, K. Srinivasa Rao, D. Narasimhan and K. Kannan
Void
Predictor coefficients
Gain
Pitch period
Voiced/unvoiced switch
Total
Overall bit rate
Number of bits per frame
18 * 8 = 144
5
6
1
156
50 * 156 = 7800 bits / second
Table 4: Bit rate for plain LPC vocoder
Void
Predictor coefficients
Gain
DCT coefficients
Total
Overall bit rate
Number of bits per frame
18 * 8 = 144
5
40 * 4 = 160
309
50 * 309 = 15450 bits / second
Table 5: Bit rate for voice-excited LPC vocoder with DCT
7
Conclusion
It is observed from voice excited LPC with Wavesurfer tool, there is a variation in
magnitude of the letter ‘Zha’ among different people. To strengthen the results, more
samples could collected from TamilNadu, Srilanka and Malasiya infuture. For this
analysis, synthesis and numerous simulations are needed. The synthesis is based on
Hidden Markov Model(HMM). Further research may be carried out by using HMM.
References
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Speech processing of the letter ‘zha’ in Tamil Language with LPC
505
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Received: August, 2009